Why is the Large plan not included in the 1st month free?

The unlimited (large) subscription cannot be used with the trial.
That’s because the major cost of the service is the amount of Neural Machine Translation API we pay for and the Large plan has no limit.

What is recommended is using the trial with a few languages and limiting the translation to a certain part of your content, using a global rule. You can, for example, exclude your blog pages from translation.

Here is a documentation about how to add rules: